The flue gases of a boiler include various gaseous impurities as a result of combustion, such as sulphur and nitrogen oxides and carbon dioxide. The present environmental protection regulations require that flue gases are cleaned and impurities are removed from them prior to releasing flue gases into the environment from a combustion plant. For this purpose, there are several cleaning methods for flue gases that can be selected, for example dry, semidry and wet methods.
In wet cleaning methods flue gases are washed with a solution-like reagent, which reacts with the impurities in the flue gases. The reaction results in compounds, which are not harmful to the environment and which can be removed from the scrubber and taken, for example, to the waste storage area of the plant.
Carbon dioxide is one of the so-called greenhouse gases, which cause climate changes. Most of the carbon dioxide emissions are created in the combustion of fossil fuels. On the other hand, there are several usages of carbon dioxide: it is, for example, used in improving oil pumping and in the food industry. Removal and recovery of carbon dioxide from flue gases is therefore not only advantageous for the protection of environment, but also enables the commercial utilization of the recovered carbon dioxide.
Carbon dioxide can be removed from flue gases by washing them with a solution containing a reagent that reacts with carbon dioxide. The removal of carbon dioxide from flue gases is, however, difficult, which is due to the fact that the flue gases created in combustion plants, especially in industrial combustion processes, often contain sulphur oxides, which hinder the separation of carbon dioxide form flue gases. This problem has been attempted to be solved by washing the flue gases in two stages, i.e. by first removing the harmful sulphur oxides and after that the carbon dioxide.
This kind of methods are disclosed, for example, in U.S. Pat. No. 6,399,030 and US 2004/1253159. In the methods disclosed in these publications amine-based reagents are used as washing solution in different washing stages. A problem with the use of amine-based washing solutions is that the reaction products from the washing, which go to the waste treatment of the plant, are difficult to process. They cannot be taken to the plant's waste storage area nor to wastewater treatment, because they are harmful substances and they must be carefully prevented from getting into groundwater. Virtually the only disposal way for amine-based reaction products is combustion. In addition, amine-based reagents are expensive. If the flue gases to be cleaned contain a great deal of impurities, the use of amine-based washing solutions is not economical, because cleaning such flue gases uses a great deal of amine reagent.
From U.S. Pat. No. 4,510,124 it is known to remove carbon dioxide from sulphur dioxide containing flue gases by washing the flue gases in one stage with a potassium carbonate solution. The used washing solution is regenerated in order to create a usable washing solution, in which regeneration the carbon dioxide separated from the flue gases is used. The problem with this method is expressly the massive washing solution regeneration process it requires. In addition, the carbon dioxide recovered form the flue gases is used entirely in regenerating the washing solution, and none of it is left to be utilized elsewhere, for example for selling.